The partial nitrification reactor was successfully started up and operated stably for more than 250 days with a maximum nitrite production rate of 1.12 kg-N m(-3) day(-1). The important factors for successful partial nitrification were high ammonium loading rate (>1.0 kg-N m(-3) day(-1)) and relatively high pH (ca. 8.0), giving high free ammonia concentrations (>10 mg NH(3)-N L(-1)). In addition, the air flow rate must be controlled at the ratio of air flow rate to ammonium loading rate below 0.1 (m(air)(3) day(-1))/(kg-N m(-3) day(-1)). After the establishment of stable partial nitrification, the effluent NO(2)(-)-N/NH(4)(+)-N ratio and effluent NO(3)(-)-N concentration were 1.20 +/- 0.33 and 1.2 +/- 1.0 mg-N L(-1), respectively, which was then fed into an granular-sludge anammox reactor. Consistent nitrogen removal was achieved for more than 250 days with a maximum nitrogen removal rate of 15.0 kg-TN m(-3) day(-1). (C) 2011 Elsevier Ltd. All rights reserved.